Current limiting protection for contactors



' saves! June 10, 1958 w. H. EDMUNDS 2,838,718

CURRENT LIMITING PROTECTION FOR CONTACTORS Filed May 28, 1953 2 Sheets-Sheet 1 June 10, 1958 w. H. EDMUNDS CURRENT LIMITING PROTECTION FOR CONTACTORS Filed May 28, 1953 2 Sheets-Sheet 2 INVEN TOR. ZI/ LL/ IM #vaaw Jana/Na:

Airman/w y? CURRENT LTMITING PROTECTION FOR CONTACTORS William Harold Edmunds, Havel-town, Pa., assignor to I-T-E Circuit Breaker Company, Philadelphia, Pa.

Application May 28, 1953, Serial No. 358,157

6 Claims. (Cl. 317-46) My present invention relates to contactors incorporating current limiting means and is more particularly directed to a contactor unit which is eifective to interrupt severe short circuit currents without the necessity of back up circuit breakers.

In the prior art contactor, the cooperating contacts were designed for an interrupting capacity of from 10 to 12 times the normal current. These contacts are nor mally biased open and are provided with a holding magnet, which is efiective when energized to close the contacts and thereby connect the load such as a motor to the source of electrical energy.

The energizing circuit for the holding magnet is connected to the source side of the contactor and has a plurality of series contacts located therein.

With the exception of the starting button, the remaining series contacts are normally biased closed. Thus, upon movement of the closing button from the open to closed position, the holding magnet will be energized thereby moving the series contacts to the closed position and effecting energization of the load.

Time delay current responsive means are inserted in the various phases of the contactor, each associated with a particular series switch in the holding magnet circuit, which are effective to open the series contacts on the occurrence of an over current to thereby interrupt the energization of the holding magnet.

Some prior art contactors have also been provided with instantaneous trip means to eitect the energization of the holding magnet on an occurrence of an overcurrent. However, even with the instantaneous trip means in the prior art arrangements, the interrupting capacity of the main contacts never exceeded 10 to 12 times normal load current and hence, it is always necessary to back up the contactor with automatic protective means such as a circuit breaker. That is, even though the contact was provided with an instantaneous trip unit in order to keep the expense of the contactor unit at a minimum by eliminating arc extinguishing means and the like, the interrupting capacity of the main contacts is kept at a minimum with reliance on a back up circuit breaker to protect the load.

Thus, although the prior art contactor has the advantage of being relatively inexpensive and simple in construction, it has the main disadvantage of necessitating a relatively expensive automatic protective equipment to be used in coordination therewith to interrupt severe short circuit currents.

In the contactor of my invention, I provide a novel arrangement wherein the capacity of the main contacts is relatively small, for example 10 to 12 times the normal load current, as used in the prior art arrangement, without arc extinguishing means but also providing means which will permit the contactor to successfully interrupt severe short circuit currents Without the necessity of a back up circuit breaker. This is accomplished by utilizing a plunger type current limiting device in each phase nited States Patent of the contactor which will be associated with a microswitch in series with the holding magnet of the contactor.

The current limiting device consists of a fuse which melts during the initial rapid rise of severe short circuit currents and thereafter releases a spring biased plunger to open the micro-switch with which it is associated and thereby interrupt the energizing circuit for the holding magnet.

The above mentioned series micro-switches associated with the time delay current responsive means are in series with the last mentioned micro-switches associated with the current limiting devices. Thus, on the occurrence of a small over current, a time delay current responsive means will open its associated microswitch thereby interrupting the energizing circuit for the holding magnet. In the event a severe short circuit occurs, the current limiting device will rupture thereby permitting its spring biased plunger to interrupt its associated micro-switch and thus interrupt the energizing circuit for the holding magnet.

Since the current limiting device operates its complete function within less than a quarter of a cycle, the full current will never be able to reach its maximum value. Since the current limiting device anticipates the occurrence of a severe short circuit current but does not permit the current to reach its maximum value, the hold ing magnet will be deenergized within the short period of time thereby permitting opening of the main contacts before the completion of a quarter of the cycle.

Accordingly, the interrupting capacity of these main contacts need not be more than 10 to 12 times normal load current due to their coordination with the current limiting device.

Thus, the advantages of the prior art contactor such as the elimination of the necessity of arc interrupting means, are maintained in my novel contactor and additional advantages are achieved by enabling the device to successfully interrupt severe short circuit current to thereby eliminate the necessity for automatic back up circuit protective means.

My copending application Ser. No. 316,221, entitled Protective Device With Current Limiting Means, filed October 22, 1952, discloses a device in which current limiting devices are incorporated in and coordinated with an automatic circuit breaker.

Although the apparatus of my copending application achieves many of the advantages of my present invention, the two devices operate on substantially different mechanical and electrical principles.

In my present device, the necessity for latches on the main cooperating contacts is eliminated as they are held in the engaged position by means of a holding magnet. Since the contactor does not use a latch type operating mechanism for the main contacts, the hammer blow effect required for the current limiting device is eliminated since it is only necessary to have sufiicient force to operate a series connected micro-switch to interrupt the energize.- tion of the holding magnet.

Thus, whereas the apparatus of my copending application utilizes a common tripper bar which necessitates a hammer blow thereon by the current limiting device, my present invention operates on an electrical principle rather than mechanical since tripper bars are eliminated. Furthermore, my present invention incorporates the features of an undervoltage device which is an inherent characteristic of the holding magnet.

Accordingly, a primary object of my invention is to provide a novel contactor which is capable of interrupting severe values of short circuit current although the interrupting capacity of the main contacts does not exceed approximately 12 times the normal load current.

Another object of my invention is to provide contactor apparatus which is self-containing and capable of interrupting all values of fault current without the necessity of automatic back up circuit protective equipment.

-A still further object of my invention is the utilization of current limiting devices which will effectively interrupt each phase of the electrical line on the occurrence of a fault in any one phase thereby preventing single ph a'sing.

Another object of my invention is to provide a novel application for current limiting devices which will prevent single phasing without the necessity of a tripper bar. A still further object of my invention is to provide a novel electrical unit which is capable of interrupting severe values of short circuit current and does not require either a latched mechanism or a tripper bar.

Still another object of my invention is to provide a novel adaption arrangement for contactors which have an interrupting capacity of 10 to 12 times normal load current so that they will be effective to interrupt large magnitudes of fault current without the need of are extinguishing means.

Another object of my invention is to provide a contact-or which can interrupt over currents due to time dclay current responsive to interrupt fault currents due to the rupture of current limiting devices and will interrupt the circuit on the occurrence of undervoltnge conditions due to the inherent conditions of the holding magnet.

These and other objects of my invention will be apparent from the following description when taken in connection withthe drawings in which:

Figure l is a schematic wiring diagram of my novel contactors incorporating the current limiting devices in each phase thereof. This figure illustrates the position of the various components when the contactor has connected the electrical source to the load and normal conditions exist on the line.

Figure 2 is a schematic wiring diagram of my novel contactor incorporating current limiting devices and illustrates the position of the main contacts when a current limiting device in any one phase has been ruptured due to the occurrence of severe short circuit currents.

Figure 3 is a side cross-sectional view of the type of current limiting device which will be used in my invention. 7

Figure 4 is a time-current curve showing the operative response of the current limiting fuse and time delay current responsive means. This figure illustrates both the inverse time characteristics of the two current responsive devices and also illustrates the manner in which they are coordinated in accordance with the magnitude of the current fault flowing through the circuit.

Figure 5 is a current-time curve illustrating time and magnitude of current at'which the current limiting device ruptures and effects interruption for the lo'ad circuit.

The current limiting device used in the cont actor of my invention may be of the type set forth in Patents 2,321,711; 2,342,310; 2,358,676 or 2,592,399.

. The current limiting device per se forms no part of my present invention but will be described to best illustrate and understand its use in my novel contacto-r.

A current limiting device 22, as shown in Figure 3, is electrically connected in series with each phase of the circuit, as seen in Figures 2 and 3. When a fault current occurs, the fusible element 29 of the current limiting device 22 is ruptured and the spring mechanism 42 is released to drive the plunger 26 towards the left. The plunger 26 will then engage the normally closed microswitch 50 thereby interrupting the energizingcircuit for the holding magnet 51.

As seen best in the cross-sectional detailed view of Figure 3, the current limiting device 22 has a hollow cylindrical housing 27 which serves as a main insulating casing for the unit. The metallic cap 27 is provided to fit over one end of the hollow cylindrical insulator 27.

The main portion of the current limiting device 22 is the fusible element 29 which is made of silver or any other metal with a high melting point. The metal cap 31) is provided to fit over the right hand end. The caps 28 and 30 are made of conducting materials and serve as terminals for the current limiting device 22 as best seen in Figures 1 and 2.

Fusi'ble element 29 has extending areas 31, 32, 33 and 34 which protrude over and around the surface of the main casing 27. When the caps 28 and 39 are placed over the hollowinsulating cylinder 27, they engage the extremities of the areas 31 and 32-33 and 34, respectively, to thereby grasp these units between the inner surface of the caps and the outer surface of the cylinder 27.

Although it has been found desirable and practical to construct a fusible element 29 with protrusion or extension areas 31, 32,33 and 34, etc., it will be understood that any other means may be provided to hold this element within the casing 27 such as the method set forth in the above mentioned patents.

The fusible element 29 is provided with a series of holes 26 laterally disposed across the element. Thus, the current will flow from the terminal cap 29 through the protrusions 31, 32 through the portion 35 and thence, through the reduced cross-sectional area 36 to portion 37, through protrusion or areas 33, 34 to the conducting cap 30. V

When a severe fault current occurs the increased resistance due to the reduced cross-sectional areas 36 located between the holes 36' will cause the fusible element 29 to rupture along the lateral line, distinguished by the holes 36.

V The conducting cap or terminal 28 has an opening 33 in the center thereof through which the biased plunger 26 extends. The fusible element 29 has a T-shaped extension 39, 39' at the left end thereof and the plunger 29 is rigidly secured thereto. The disc 49, positioned within the cap2'3 by means of the hollow cylindrical insert 45 and fractionally held therein, has an opening 41 in the center thereof.

The insulating disc spacer 4i acts as a barrier for gas which may be blown within the current limiting device and also S6l'VS as a base for the compression spring 42. The compression spring 42 is disposed between the in sulating disc spacer 4t and the collar 39. The collar 39' thus acts as a second base for the compression spring 42 and also acts as a closure for the opening 33 in the metallic terminal cap 28 to prevent explosion of gases when the fuse 29 is ruptured.

Thus, on the occurrence of a fault current, the fuse 29 will rupture while the lateral line distinguished by the holes 36' and the compressed spring 42 will thereafter be instrumental in driving the collar 36 and the plunger 27 to the left to thereby engage the micro-switch 50 associated with the current limiting device.

The fusible element 29 is surrounded by a powdered quartz filler 43 which is vitrous material. Thus, when the fusible element 29 is ruptured due to excess of heat caused by the fault current, silver vapors will be formed within the housing 27 to aid in the extinguishing of the are which will gap 2. portion 35, 37 of the fusible element 29. Since the distance of separation of the two parts 35, 37 of the fusible element 29 is not sufliciently large to prevent restriking of the are, it is necessary to supply supplemental means to prevent restriking. This is accomplished as follows:

The heat generated by the fusible element 29 is sufficiently large to vitrify the powdered quartz filler 43 thereby melting same to form a glass barrier between the portions 35 and 37 of the fusible element 29. Hence, a solid glass insulator is formed by this operation to render restriking of the are impossible.

As noted, the silver vapor formed by the fusible link 29 aids in the extinguishing of the are. Thus, it is desirable to build up as large a pressure within the current limiting device as possible. This is achieved by means of the collar 39' which following rupture of the element 29 will be urged to the left to thereby seal off the opening 38 of the metal cap terminal 28 to thereby enable sufficient pressure of the vapors to extinguish the fault current are.

In like manner, the insulator disc spacer 40 acts as a barrier for the gas vapors to aid the collar 39 in maintaining a high pressure.

The contactor if Figures 1 and 2 is connected between the source 57 and the load 52. In the illustration shown, a three phase line a, b and c is illustrated and hence components in each phase are designated by identical numerals followed by the letter a, b or c of the phase with which they are associated.

The main contacts 53 are operated from a common shaft or armature 54 and biased in the open position by means of spring 55 which is attached to the stationary connection 56. The arrows associated with the spring illustrates the direction of bias.

When the holding coil 51 is deenergized, the spring 55 will bias the contacts to the position indicated in Figure 2. Upon energization of the holding coil 51, the main'contacts 53 will be moved to the closed position indicated in Figure 1. The time delay current responsive means 19 may be bimetallic elements with a calibration screw 25 threaded in the free end thereof.

In the illustrations of Figures 1 and 2, a time delay current responsive means is shown for phases a and c. However, it will be apparent to those skilled in the art that phase b may also be provided with same.

Current limiting device 22 is electrically connected in series with the main contacts 53 in a time delay current responsive means 19. Thus, current flow through the circuit for example through phase a will be from the source 57 through main contacts 53:; to the bimetallic element 19a and the current limiting device 22a to the load 52.

The time delay current responsive means 19a is connected to the circuit by means of the pigtail 20 and has a micro-switch 66 associated therewith. The microswitch 60 is biased closed by means of the compression spring 61. On the occurrence of an overcurrent, the bimetallic element 18 will deflect to the right, as indicated by the dotted line of Figure 2, to thereby open the micro-switch 60 and interrupt the energizing circuit for the holding magnet 51.

Each of the current limiting devices 22 is associated with a micro-switch 50 which is biased closed by means of the spring 63. It wil be noted that the three series connected micro-switches 59a, 50b and 500, which are in series with the micro-switches 60a and 600, may be of similar construction.

On the occurence of a fault current which is greater than a mere over current, the current limiting device 22 will rupture as above noted and the plunger 26 will be driven to the left to thereby open its associated microswitch 50. The current limiting device 22 may be snapped into the circuit by mechanical and electrical connection between the clips 65 and 66 which respectively engage the metallic terminal caps 28 and 30.

Thus, in the event any one of the current limiting devices 22 should rupture, its removable and replaceable mounting in the clips 65 and 66 permits replacement of the ruptured unit by a new device.

The bimetal 19 is an L-shaped member which is permanently secured to the base 70 of the contactor by means of the screw 71 which passes through the terminal 72 and one leg of the L-shaped bimetallic member 19.

Since all of the series connected micro-switches 50 and 61} are connected in series with the holding magnet coil 51, the contactor may be operated by moving the starting but- .6 ton 74 from the dotted open position of Figure 2 to the closed position of Figure 1.

Since the energizing circuit for the holding magnet coil is in parallel with the source 57 at the source end of the contactor, it wil be energized through the plurality of normally closed series micro-switches 50 and 60. The holding magnet coil will then pull the plunger 54 downward thereby closing the main contacts 53a, 53b and 53c, and current will now flow from the source 57 to the load 52.

Since the main contacts 53 are maintained in their closed position by the holding magnet coil 51, this engagement of the contacts can be accomplished in any one of three manners. For example, if a severe fault current occurs in phase 0, the current limiting device 220 will rupture thereby permitting the compression spring 42 to drive the plunger 26 to the left. The plunger will thus open the micro-switch 500 against the bias of its closing spring 630 to thereby interrupt the energizing circuit for the holding magnet coil 51. The de-energized holding magnet coil will then relinquish control of the main contacts to the opening biasing spring 55 to thereby interrupt the circuit.

It will be noted, however, that the main contact 53c will open on a no load circuit since the current limiting device 220 has ruptured and completely interrupted the flow of current in this phase. The remaining two main con tacts 53a and 53b will open on either normal load current or a slight over current even though a severe fault current had previously existed in phase c. Thus, in spite of the relatively low interrupting capacity of the main contacts 53a, 53b, 530, the contactor will have completely interrupted the flow of current to the load 52 on the occurrence of a severe fault current.

It is also possible that the contactor can be opened due to an over current condition. Thus, for example, in the event of an overcurrent condition existing on phase C, the bimetallic elements 190, when overheated, will deflect to the left (as indicated by the dotted line in Figure 2) thereby bringing is calibration screw 250 into engagement with the micro-switch 600. The opening of the micro-switch 600 due to the deflection of the bimetallic element will thereby interrupt the energizing circuit for the holding magnet coil 51 and hence, the main contacts 53a, 53b, and 530 will be opened by the spring 55 due to the occurrence of an overcurrent.

The bimetallic elements as will hereinafter be more fully described will be effective to cause circuit interruption when the fault current is below 10 to 12 times normal load current and the current limiting device 22 will be eifective for fault currents in excess of this mag nitude. Thus, in the event that the contactor should be opened due to the deflection of one of the bimetallic elements 19, the interrupting capacity required of the main contacts 53 will not exceed 12 times the normal load current. Thus, in both operations, that is for time delay trip due to current responsive means 19 or instantaneous trip due to current limiting devices 22, the necessary interrupting capacity for the main contacts 53 1s a minimum.

A third method of opening the contactor is by the occurrence of an undervoltage. That is, since the holding magnet coil 51 maintains the main contacts 553 closed against the opening spring 55, due to its energization obtained from the voltage at the source side of the contactor, a drop in supply voltage to a predetermined value will sufiiciently reduce the ampere turns of the holding magnet coil 51 so that the opening force of the spring 55 will predominate and the main contacts will be moved to the open position shown in Figure 2.

It will also be noted that the operator can move the closing switch 74 from the closed position to the open position to thereby cause deenergization of the holding magnet coil 51 and result in opening of the contacts 53a, 53b and 530.

Thus, it will be noted that my novel contactor utilizing current limiting devices is capable of interrupting severe short circuit current even though the main contacts have a minimum interrupting capacity. Thus, the self-contained contactor does not require a back up circuit breaker to protect the load from the occurrence of severe load current.

Therefore, my novel unit incorporates both time delay and instantaneous interrupting features and prevents single phasing without the necessity of latch means, tripper bars or toggle mechanisms. That is, since the plurality of micro-switches associated with the time delay and instantaneous current responsive means are located in series in the energizing circuit of the holding coil, an overcurrent or fault condition in any one phase will be instrumental in ri e-energizing the holding magnet coil 51 to thereby result in theinterruption of the current in all of the phases.

The coordination of the instantaneous current limiting device 22 with the time delay current responsive means 19 is illustrated in Figures 4 and 5. As seen in Figure 4, the designates the normal or operating current flowing from the source 57 through the contactor to the load 52. When current of this magnitude flows through the circuit, it is insuflicient to deflect the bimetallic element thermal 19 or to rupture the current limiting device 22.

' However, if an overcurrent occurs so that a magnitude is between the values designated by i;, and i the thermal element 19 will be sufficiently deflected to bring its calibration pin 25 into engagement with its associated micro-switch 60 to thereby effect de-energization of the holding magnet coil 51.

it will be noted that during the occurrence of overcurrents between the magnitude i and i that the fusible element 29 of the current limiting device 22 will be heated. However, as is apparent from Figure 3, the deflection of the bimetallic element 19 will occur long before the element 29 is ruptured and hence, only the bimetallic 19 will be instrumental in interrupting the energizing circuit for the coil 51.

However, on the occurrence of a severe fault current above the value 1 the heat generated in the reduced cross-sectional area 36 of the fusible element 29 will be suflicient to melt the fusible element thereby permitting the compression spring 42 to drive the plunger 26 forward. Since the time required for the bimetal element 19 to open its associated micro-switch will be much greater than the time required for the fusible element 19 to melt, the rupture of the fusible element will occur first.

The inverse time characteristic of the current limiting device 22 shown in Figure 4 is illustrated in the cycle curves of Figure 5. Thus, with the short circuit current of the magnitude of the I the fusible element 27 of the current limiting device 22 ruptures when the current magnitude in'the first quarter of the cycle reaches a magnitude indicated by the numeral 60. Under these circumstances, the current limiting device 22 will interrupt the circuit in its phase at time D. However, if the short circuit current is more severe, for example of the magnitude indicated by the 1 the current will reach the magnitude indicated by the numeral 60 in a shorter period of time and hence, the fuse link 29 will rupture in a shorter period of time. Thus, since the rate of increase is more rapid on the occurrence of a short circuit current of the magnitude I the current limiting device 22 will rupture at time B to thereby interrupt the current in the phase in which it is located.

In the foregoing, I have described my invention only in connection with preferred embodiments thereof. Many variations and modifications of the principles of my invention within the scope of the description herein 8 are'obvious. Accordingly, I prefer to be bound not by the specific disclosure herein but only by the appending claims.

I claim:

1'. In a contactor; a plurality of cooperating contacts; each of said cooperating contacts being connected in series with a thermal trip device and a current limiting device; said current limiting device comprising a fusible member operatively connected to a striker pin; said fusible member being constructedto interrupt the current flowing therethrough; said current limiting device causing said striker pin to be moved responsive to operation of said fusible member; said plurality of cooperating contacts being linked together for simultaneous movement to either engaged or disengaged position; electromagnetic means for maintaining said plurality of cooperating contacts in said engaged position; said current limiting device being electrically connected to interrupt short circuit currents flowing through said pair of cooperating contacts; said thermal trip device and said current limiting device being operatively connected by mechanical means to said energizing circuit to selectively effect de-energization thereof on the occurrence of a fault current; said mechanical means for operatively connecting said current limiting device and said energizing circuit including said. striker pin of said current limiting device.

2. In combination, a polyphase contactor for controlling circuit connections from a polyphase power circuit to a load and having individual sets of contacts relatively movable with respect to each other between engaged and disengaged position, said sets of contacts being individual to each phase, biasing means for biasing said contacts to disengaged position, a closing electromagnetic means, an energizing circuit for said electromagnetic means connectible tothe power circuit and including said electromagnetic means and normally closed contacts connected in series, said closing electromagnetic means when energized operating said contacts into energized positions, fault current responsive means including a current limiting device and a thermal trip device to selectively open said energizing circuit connected individually in each phase of said polyphase circuit for responding to fault currents in the individual phases for opening said phase;

'said current limiting device comprising a fusible member operatively connected to a striker pin; said fusible member being constructed to interrupt the current flowing therethrough; said current limiting device causing said striker pin to be moved responsive to operation of said fusible member; said current limiting device electrically connected to interrupt short-circuit currents flowing through its individual phase, a mechanical member for each fault current responsive means to be operated thereby, each of said mechanical members being positioned in engageable relation with respect to said striker pin of its respective current limiting device, each mechanical member operating to open individual series connected contacts to thereby de-energize the energizing circuit of said electromagnetic means to permit said biasing means to operate all of said sets of contacts to disengaged position; said thermal trip device operatively energized for all low values of fault current and said current limiting device operatively energized for all high values of fault current.

3. In combination, a polyphase contactorfor controlling circuit connections from a polyphase power circuit to a load and having individual sets of contacts relatively movable with respect to each other between engaged and disengaged position, sets of contacts being individual to each phase, biasing means for biasing said contacts to disengaged position, a closing electromagnetic means, an energizing circuit for said electromagnetic 'means connectible to said power circuit and including said electromagnetic means and normally closed contacts, fault current responsive means including a current limiting device and a thermal trip device for opening said phase; said current limiting device comprising a fusible member operatively connected to a striker pin; said fusible member being constructed to interrupt the current flowing therethrough; said current limiting device causing said striker pin to be moved responsive to operation of said fusible member; a mechanism connected to said fault current responsive means to be operated by said fault current responsive means which responds to a fault in its phase, for operating said normally closed electromagnetic contacts to open position to open the circuit of and'de-ener gize said electromagnetic means; said current limiting device electrically connected to interrupt short-circuit currents flowing through its individual phase; said thermal trip device operatively energized for all low values of fault current and said current limiting device operatively energized for all high values of fault current; said mechanism for operatively connecting said current limiting device and said energizing circuit including said striker pin of said current limiting device.

4. In combination, a polyphase contactor for controlling circuit connections from a polyphase power circuit to a load and having individual sets of contacts relatively movable with respect to each other between engaged and disengaged position, set of contacts being individual to each phase, biasing means for biasing said contacts to disengaged position; opening means connectible to the power circuit and including an electromagnetic means for operating, when energized, said polyphase contacts into engaged positions against said biasing means; current responsive means including a current limiting device and a thermal trip device connected individually in each phase of said polyphase circuit responsive to fault currents in the individual phase for extinguishing the fault current; said current limiting device comprising a fusible member operatively connected to a striker pin; said fusible member being constructed to interrupt the current flowing therethrough; said current limiting device causing said striker pin to be moved responsive to operation of said fusible member; switch mechanism in said energizing circuit controlled by said current responsive means energized by a fault current for operating said switch mechanism to open the energizing circuit of said electromagnetic means; said current limiting device electrically connected to interrupt short-circuit currents flowing through its individual phase; said thermal trip device operatively energized for all low values of fault current and said current limiting device operatively energized for all high values of fault current; said switch mechanism operatively connecting said current limiting device and said energizing circuit including said striker pin of said current limiting device.

5. In combination, a polyphase contactor for controlling circuit connections from a polyphase power circuit to a load and having individual sets of contacts relatively movable with respect to each other between engaged and disengaged position, said sets of contacts being individual to each phase, biasing means for biasing said sets of contacts to disengaged position, a closing electro-magnetic means, an energizing circuit being connected to and energized by said polyphase circuit and including said closing electro-magnetic means; said closing electro-responsive means operating to move said sets of contacts into engaged positions against said biasing means, a thermal trip element and a current limiting device being connected individually in each phase of said polyphase circuit and in electrical series with associated sets of contacts, said thermal trip element and said current limiting device being energized respectively by fault currents below and above a predetermined value; said thermal trip element and said current limiting device being operatively connected to said energizing circuit to selectively effect deenergization of said energizing circuit on the occurrence of fault currents through their associated set of contacts; said current limiting device being effective to interrupt fault currents through its associated set of contacts prior to the opening 'of said set of contacts.

6. In combination, a polyphase contactor for controling circuit connection from a polyphase power circuit to a load and having individual sets of contacts relatively movable with respect to each other between engaged and disengaged position, said sets of contacts being individual to each phase, biasing means for biasing said sets of contacts to disengaged position, a closing electro-magnetic means, an energizing circuit being electrically connected to and energized by said polyphase circuit and including said closing electro-magnetic means for operating said sets of contacts into said engaged position against said biasing means; each of said sets of contacts being connected in electrical series with a time delay trip means and a current limiting device; said time delay trip means and said current limiting device being energized respectively by fault current below and above a predetermined value; said time delay trip means and said current limiting device being operatively connected to said energizing circuit to selectively effect de-energization of said energizing circuit on the occurrence of fault currents; said current limiting device being effective to interrupt fault currents through its associated set of contacts prior to the opening of said set of contacts upon the de-energization of said energizing circuit.

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